The present invention relates to a silencer for deadening noise produced from a refrigerant compressor of a refrigeration system by the effect of sound wave interference.
Almost every home is generally furnished with refrigeration system such as a household refrigerator, which is in continuous operation throughout seasons. Such a household refrigerator has. In the refrigerator, one critical noise source is a machine compartment enclosing a compressor and piping system connected to the compressor. More specifically, from the machine compartment is emanating relatively loud noise, for example, noise produced with drive of a compressor motor, noise produced with flowing of the compressed gas, and mechanical noise produced by movable members of a compression mechanism. Further, the piping system connected to the compressor produces noise due to vibration thereof. The noise emanating from the machine compartment thus accounts for a large part of noise of the refrigerator. Accordingly, control of noise from the machine compartment contributes to noise reduction in the refrigerator.
Conventionally, compressors of the low noise type such as a rotary compressor have been employed for the purpose of reducing noise emanating from the machine compartment. Further, the construction of vibration-proofing of the compressor has been improved and the configuration of the piping has been improved, thereby providing damping of vibration in a vibration transmission path. Further, noise absorptive and insulative members have been disposed around the compressor and piping system, thereby improving an amount of noise absorbed in the machine compartment a noise transmission loss.
However, a plurality of ventilating openings are formed in one or more of walls defining the machine compartment for ventilating the machine compartment, and the noise produced in the machine compartment is caused to leak outward through the ventilating openings. As the result of provision of the ventilating openings, the above-mentioned conventional noise-reduction methods each have a definite limit and provide the noise reduction of 2 dB (A) at the most.
On the other hand, with advancement of applied electronics technique including sound data processing circuitry and acoustic control technique, application of a noise control wherein noise is deadened by the effect of sound wave interference has recently been taken into consideration. More specifically, in the above-mentioned noise control, sound generated by a noise source is received by a sound receiver such as a microphone disposed in a specific position and the sound receiver generates an electrical signal in accordance with the received sound. The electrical signal is then converted to a control signal by signal converting means. The control signal is supplied to a speaker so that an artificial sound of opposite phase or 180.degree. out of phase with the noise received by the microphone and having the frequencies same as those and the amplitude same as that of the received sound is produced by the speaker, so that the artificial sound interferes with the received sound, thereby deadening the sound.
However, when such a noise control is applied to the refrigeration system such as a household refrigerator, the following circumstances peculiar to the refrigeration system needs to be taken into account. That is, energization and deenergization of the compressor are alternately reiterated with increase and decrease of the storage compartment temperature. At the starting of the compressor, particularly, the revolution of the compressor motor is rapidly increased from 0 to, for example, 3,600 rpm. in several hundredths of seconds. With such a rapid increase in revolution, the noise level is instantaneously increased a large extent. Thereafter, the noise level is decreased as the revolution is stabilized, as shown in FIG. 7. Since the sound pressure of noise is low and stabilized in the normal running after starting, sufficient noise reduction may be achieved by the noise control employing the feedback control system. However, when the noise level itself is high and rapidly increased to a large extent as in the starting of the compressor motor, a processing period from detection of noise by the receiver to completion of the processing causes the timing of producing an artificial sound to slightly lag behind. Although such a timing lag may be ignored in the normal running of the compressor, it increases the difference between the noise and artificial sound. Consequently, sufficient noise reduction cannot be achieved in the starting of the compressor motor.